Why can't the ship be partially constructed on earth and then raised into orbit and launched from the "SF Shipyard" after it's internals are completed?
Isn't there a scene in TNG that shows a Galaxy (or similar) class being built on the ground from orbit?
There's a picture of pieces of the ship laying on the ground from a sattelite imagery in "Paralells."
It suggests the ship was built in pieces on Mars' surface (which would have the similar low-G and atmospheric hurtles of orbital construction.) Not that the entire ship was built on the ground in one fell swoop.
And for all we know it could be a "junkyard" for discarded components or a storage lot for components to be tractored in space or, hell, the pieces could be in orbit with the gound just being in the background.